This paper presents in situ tests which were performed on both single piles and pile groups, loaded to failure, with the aim of studying the pile group effect of piles embedded in multi-layered foundation. Strain gauges were installed along the shaft of 10 m long steel pipe piles, with a diameter of 143 mm. The influence of loose sand layers on the group effect in case of friction piles was evaluated. The experimental results indicated that the influence of sand layers was evident, and the group factor was calculated to be 1.237.
Journal of Science and Technology in Civil Engineering NUCE 2019 13 (3): 135–142 EXPERIMENTAL TESTING OF A FULL-SCALE OF GROUP EFFICIENCY IN MULTIPLE SOIL LAYERS Le Thiet Trunga,∗, Duong Diep Thuyb , Pham Viet Anhb a Faculty of Bridge and Highway Engineering, National University of Civil Engineering, 55 Giai Phong road, Hai Ba Trung district, Hanoi, Vietnam b Faculty of Information Technology, National University of Civil Engineering, 55 Giai Phong road, Hai Ba Trung district, Hanoi, Vietnam Article history: Received 30/07/2019, Revised 27/08/2019, Accepted 28/08/2019 Abstract Results of in-situ tests showed that the performance of single isolated piles and individual piles within a group is largely different When piles are arranged in a group, the interaction between piles and the foundation depends on the pile arrangement and the pile group effect To date, studies on the pile group effect in Vietnam have been limited to reduced-scale laboratory testing or static load testing where piles are installed into homogeneous sandy or clayey foundation This paper presents in situ tests which were performed on both single piles and pile groups, loaded to failure, with the aim of studying the pile group effect of piles embedded in multi-layered foundation Strain gauges were installed along the shaft of 10 m long steel pipe piles, with a diameter of 143 mm The influence of loose sand layers on the group effect in case of friction piles was evaluated The experimental results indicated that the influence of sand layers was evident, and the group factor was calculated to be 1.237 Keywords: group efficiency; pile groups; axial capacity; load transfer https://doi.org/10.31814/stce.nuce2019-13(3)-13 c 2019 National University of Civil Engineering Introduction Pile group effect is the effect of mutual interaction between piles in a group through the foundation, having influence on the general load capacity of the pile group The interaction between piles in a group can result in two types of effect (1) to alter (mainly reduce) the load capacity of the entire group compared to the total load capacity of individual piles; (2) group effect, increasing the stress transmission area, causing the settlement of the pile group much higher than the single pile, especially when there is a soft soil layer near the pile tip Pile group effect appears very differently between different types of piles and different types of soil The influence of the group effect is often shown clearly in the friction pile group when the piles are close to each other and the soil is clayey There have been many studies on pile group effect ([1–6]) that studied the subsidence of pile group, based on various approaches, including the methods of boundary element, load transfer and finite element There has been much progress in analyzing and predicting the pile group effect over the past few decades, but the analysis is still largely based on simplification of the problem and behavior of the soil Due to the difficulties and costs of 1:1 model load test, most pile group tests have been performed on miniature models in situ or in the laboratory [7] ∗ Corresponding author E-mail address: lethiettrung@gmail.com (Trung, L T.) 135 Trung, L T., et al / Journal of Science and Technology in Civil Engineering Ismael [8] studied behavior of bored pile groups in cement sand by in-situ tests The compression tests were conducted on two pile groups, each group consists of five piles The spacing of piles in the groups was two and three diameters The pile group effect determined from the tests was 1.22 and 1.93, corresponding to the pile spacing of two and three times of the pile diameter Al-Mhaidib [9] studied the behavior of pile groups in sandy foundation under different load levels A total of 60 tests were conducted in the laboratory, using a model of small diameter steel piles, placed in medium tight sand The sample piles had an outer diameter of 25 mm and a length of 500 mm in the soil Five ways of arrangement of piles in the group (2×1, 3×1, 2×2, 2×3, 3×3) with the pile spacing 3d, 6d and 9d (d is the pile diameter) were tested Deb and Pal [10] carried out testing on 1×1, 2×2 and 3×3 pile group models, with the ratio of length in soil to diameter (L/D) of 5, 6, 7, and the distance between the piles equalling times of the diameter, subject to vertical load The sample piles used for testing had a length of 250 mm with a diameter of 25 mm Elsamny et al [11] studied load bearing capacity and behavior of piles in sandy foundation for single piles and group of two piles under axial compressive load Experimental research was carried out to study axial friction distribution and load on pile heads, in non-clayey soil (tight sand), as well as group effect of two piles The distance between piles was three times of the pile diameter The calculated pile group effect was 1.43 Yudiawati et al [12] studied pile group effect and the efficiency of friction piles on very weak clay foundation, which were performed by static compression of friction piles in very soft clay The test piles consisted of square concrete piles of 20 cm × 20 cm, with the length of 11.5 m and 17.5 m The pile groups included 2×2 piles and 3×3 piles, with spacing between piles of to times of the pile diameter Load tests were conducted on single piles (2 experiments), on individual piles in a group of piles (5 experiments) and on a group of piles (4 experiments) The results showed that load bearing capacity of a single pile, in general, was higher than capacity of individual pile in a pile group, so the coefficient of pile group effect found ranged from 50% to 70% According to Tomlinson and Woodward [13], axial load capacity of a pile group may be significantly smaller than total load capacity of individual piles in a group In all cases, elastic and consolidation settlement of a pile group is larger than the settlement of a pile that bears the same load as on individual pile in a group This is because the subsidence area of the pile group extends to a wider width and depth than the area below the single pile In previous studies, experimental studies were mainly conducted with homogeneous foundation, loose soil or cohesive soil In the scope of the paper studying the pile group effect in case of multilayered foundation, especially the effect of porous, medium-tight sand layers in the foundation on the load bearing capacity and subsidence of friction piles, the following issues are clarified: (i) coefficient of pile group in multi-layered foundation, with sand and clayey soil layer; (ii) distribution of forces along the pile shaft; (iii) effect of pile group on settlement of the pile group Testing models In this study, static load tests are carried out on natural ground at the 2nd campus of National University of Civil Engineering in Phu Ly city, Ha Nam province Testing models include: (i) static load test for single piles; (ii) static load test for a group of piles with spacing of pile center - pile center ∆ = 3.5D (D is pile diameter) All tested piles are steel pipe piles with outer diameter (D) of 143 mm, thickness (δ) of mm, all piles have the same length (LPile ) of 10 m, steel strength (E0 ) of 2.1 kN/cm2 for both piles and pile caps Single pile testing area is about m away from the testing 136 The first test was carried out 10 days after the initial pile driving Tests are carried out according to the slow loading cycle (according to the Vietnamese standards TCVN 9393:2012), including cycles, cycle 1, loading to 100% of the predicted load, cycle 2, Trung, L T., et al / Journal of Science and Technology in Civil Engineering loading to destructive one Load increasing level is adjusted according to the actual test load is affected the single meter piles readsare exactly to 0.01 Settlement area The of pile group (Fig 1).by In hydraulic this study, jacks, tests with denoted by N◦mm 1, tests with a group ◦ ◦ of is piles are denoted bypositions N 4, the index N indicates piledisplacement in that group.sensors measured at four on theafter top surface of the alidspecific with four Figure Arrangement test piles, (i) single piles, (ii) group of 04 piles, survey drilling position Figure Arrangement test piles, (i) single piles, (ii) group of 04 piles, survey drilling position Stratum in the testing area is determined from the geological survey results A hole is drilled to a depth of 20 m at the testing area Standard penetration tests (SPT) and laboratory tests have been carried out to determine the mechanical and physical properties of soil Drilling results show that strata consist of the following soil layers: Layer 1, covering soil, leveling soil, composition and status are heterogeneous, compacted, 3.0 m thick; Layer 2, clay mixed with organic matter, brown gray, mahogany brown, quasi-plastic state, 1.0 m thick, SPT (NT B ) = 4; Layer 3, dust, ash gray, brown gray, porous texture, 2.7 m thick, SPT (NT B ) = 8; Layer 4, brown-gray clay, mahogany brown, dark gray, quasi-plastic state, 1.4 m thick, SPT (NT B ) = 4; Layer 5, clay mixed with sand, brown gray, mahogany brown, quasi-plastic state, 9.9 m thick, SPT (NT B ) = Underground water level is 0.7 m deep With the pile length of 10 m, the strata that piles go through include layers of soil surveyed Tests are carried out to study group effect, in the condition of multi-layered foundation, considering the effect of sand layers in the foundation The piles are lowered by static jacking method A steel plate of 0.7 m × 0.7 m dimension, cm thick is placed on single piles and pile group as pile cap Fig shows a layout of piles, single piles and pile group The pile caps are located on the ground (not in contact with the ground), and can be considered absolutely hard The axial load is transmitted along the piles, measured by strain gauges, uniformly installed on each section for all test piles The gauge used in this study is Strain Gauges FLA-5-11 Fig illustrates the pile structure, the location for installing strain gauge along the pile shaft The first test was carried out 10 days after the initial pile driving Tests are carried out according to the slow loading cycle (according to the Vietnamese standards TCVN 9393:2012), including cycles, cycle 1, loading to 100% of the predicted load, cycle 2, loading to destructive one Load increasing level is adjusted according to the actual test The load is affected by hydraulic jacks, the meter reads exactly to 0.01 mm Settlement is measured at four positions on the top surface of the lid with four displacement sensors 137 Construction science and technology magazine of NUCE 2019 Trung, L T., et al / Journal of Science and Technology in Civil Engineering Construction science and technology magazine of NUCE 2019 2019 Construction science and technology magazine of NUCE compressing up to and and increasing the the loadload of the second cyclecycle compressing up 30 to T, 30 unloading T, unloading increasing of the second compressing to destructive The The test results for 4-pile group showshow that,that, withwith a load of of compressing to destructive test results for 4-pile group a load 49.549.5 T, the group has has a destructive phenomenon, the settlement has has increased T, pile the pile group a destructive phenomenon, the settlement increased suddenly to 35.59 load-settlement curves for single (cycle 1) group and group suddenly to 35.59 mm.mm TwoTwo load-settlement curves for single pilespiles (cycle 1) and piles (cycle 2) relatively are relatively similar, increase slowly; curve a small of of piles (cycle 2) are similar, bothboth increase slowly; P-S P-S curve has ahas small angular coefficient before destructive angular coefficient before destructive load.load Limited capacity for single (PUltimate ) isT,10for T,4-pile for 4-pile group (PUltimate Limited load load capacity for single pilespiles (PUltimate group (PUltimate N°110 N°1) is pile group coefficient is 1.2375 The settlement of single piles group coefficient is 1.2375 The settlement of single piles Figure2.2.Geological Geological pillar area, pilepile structure, position of displacement sensor Figure pillarofoftesting testing area, structure, position of displacement is smaller of pile the pile group as destructive, values of settlement is smaller than than of the group as destructive, thesethese two two values of settlement are are sensor recorded as relatively (31.15 and 35.59 which is also consistent recorded as relatively largelarge (31.15 mm mm and 35.59 mm),mm), which is also consistent withwith the the Result analysis characteristics of friction piles (it is not good for pile tip to rest on the soil layer) characteristics Result analysis of friction piles (it is not good for pile tip to rest on the soil layer) Normally for friction in cohesive the group pile group coefficient is usually smaller Normally for friction piles,piles, in cohesive soil, soil, the pile coefficient is usually smaller Test results include (1)insubsidence corresponding each load level, expressed by Test include corresponding to each load level, expressed by than 1, however in(1) this subsidence case of testing, even ifto is a friction pile, due toinfluence the influence of thanresults 1, however this case of testing, even if it is ait friction pile, due to the of load-settlement curveload-settlement (P-S) for single piles and group of piles; (2) axial strain is measured by strain gauges the (P-S) pile group is 1.2375 canassessed be(2) assessed sandsand layer,layer, the pile group coefficient is 1.2375 (greater thanthan 1) be that that curve forcoefficient single piles and(greater group ofIt1) 4canItpiles; axial strain is with each effect load level,layer represented by the load distribution curve according to capacity the capacity depth (F-z) These sandy soil layer and porous sand layer (layer 3) have affected the load bearing sandy and porous (layer 3) have affected the load bearing measured bysoilstrain gauges sand withlayer each effect load level, represented by the load results are shown in 3,This and ofpile theFigs pile group This will be1 more evident inresults the results of load distribution by depth of the group willTable be more evident in the of load distribution by depth ) is 49.5 T, calculated T, calculated pile N°449.5 N°4) is distribution curve according to the depth (F-z) These results are shown in Figs and Load Load (T) (T) Load Load (T) (T) 4, Table 0 0 20 20 40 40 60 60 5 10 10 15 15 0 0 Settlement (mm) Settlement (mm) Settlement (mm) Settlement (mm) Fig 5shows the 405 load-settlement curve (P-S)5for single pile (Figure 3.a) and group 5.405 2.2052.565 2.2052.565 P-S P-S 4.2704.270 of piles 10 N°410(Figure 3(b)) For single piles, 10 is carried out with the first cycle 10 test compressing limited bearing capacity of the pile The 15 15 destructive to determine the 15 15 to 20 20 20 20 capacity of a pile is the maximum ultimate bearing load which it can carry without 25 25 25 25 failure or excessive settlement of the ground The test results show that when the test 30 30 30 30 31.150 31.150 load increases to 10 T, the settlement of the piles increases suddenly to 31.15 mm, the 35 35 35 35 P - SP - S piles are destroyed This limited load is used 40 to adjust the load level in testing a group 40 40 40 ◦is N°1 ◦ of piles The group of piles loading cycles, the (a) Test for 4single piles (b) Test for 44with piles N°4 (a) Test for single single piles N°1 (b) 2Test for group of of piles N°4 N (a) Test for piles N tested with (b) Test for group group of piles first cycle Figure Load-settlement curve (P-S) Figure Load-settlement curve (P-S) Figure 3.3.Load-settlement curve (P-S) 138 shows the axial transmission in single and pile groups Fig Fig shows the axial loadload transmission curvecurve (F-z)(F-z) in single pilespiles and pile groups during the loading process to some objective subjective reasons during the loading process Due Due to some objective and and subjective reasons whenwhen installing the gauge and lowering the piles, gauges errors during the and test and installing the gauge and lowering the piles, somesome gauges havehave errors during the test failmeasure to measure the deformation of piles (at position the position m single for single fail to the deformation of piles (at the of of m 9for pilespiles N°1,N°1, position of m 3.9formpiles for piles C1, N°4 C2 position and position of m 6.5formpiles for piles position of 3.9 N°4 N°4 C1, N°4 C2 and of 6.5 N°4 N°4 C3),C3), in in which indicates that most of the load has been distributed through friction into the soil These results show the tip resistance has been mobilized to a minimum (near zero), and These results show the tip resistance has been mobilized to a minimum (near zero), and the load imposed on the pile head is distributed (subjected to) by the axial friction the load imposed on the pile head is distributed (subjected to) by the axial friction resistance component In In thethe depth from to 3.93.9 m,m, thethe F-z curve resistance component depth from to F-z curvehas hasthe thelargest largestslope, slope, the the slope decreases at the area from 3.9 m to m, indicating that friction resistance slope decreases at the area from 3.9 m to m, indicating that friction resistanceisis Trung, L.atT.,the et area al / Journal Science mobilized much from of to 3.93.9 m.m.and Technology in Civil Engineering mobilized much at the area from to Axial load transmission F-z N°1 Axial load transmission F-z N°1 Axial load transmission F-zF-z N°4 C1C1 Axial load transmission N°4 Load F F(kN) Load (kN) 0 20 20 40 40 60 60 80 80 100100120120 0 20 20 4040 6060 8080 100 100 120 120 140 140 0 1 1 2 2 3 3 4 4 5 6 7 7 8 8 9 10 10 Depth z (m) Depth z (m) Depth z (m) Depth z (m) Load F (kN) Load F (kN) 5 6 9 Construction science technology magazine of NUCE Construction science andand technology magazine of NUCE 20192019 10 10 ◦ (a) For single piles single piles (a) (a) forfor single piles pilesN°4 N 4C1 C1 (b) for N°4 C1 (b)(b) forFor piles Axial transmission F-z N°4 Axial loadload transmission F-z N°4 C2 C2 Axial transmission F-z N°4 Axial loadload transmission F-z N°4 C3 C3 F (kN) LoadLoad F (kN) F (kN) LoadLoad F (kN) 1 2 2 3 3 4 5 5 6 Depth z (m) 6 7 7 8 8 9 9 Depth z (m) Depth z (m) Depth z (m) 20 2040 4060 6080 80 100 100 120 120 140 140 0 20 2040 4060 6080 80100 100 120 120 140 140 0 10 10 ◦ 10 10 (d) For piles N◦ C3 (c) For piles N C2 (c) (c) for for piles N°4N°4 C2 C2 piles (d) (d) for for piles N°4N°4 C3 C3 piles Figure Axial Axial load transmission curve (F-z) Figure load transmission curve (F-z) Figure Axial load transmission curve (F-z) Fig showsComparing the load-settlement curveforce (P-S) single pile (Fig 3(a)) and group of piles N◦ results of vertical offor single pilepile N°1N°1 and pile N°4N°4 C3 C3 (figures a Comparing results of vertical force of single and pile (figures a (Fig 3(b)) For single test is 3.9 carried out with the first cycle compressing to destructive andand d), at the depth of 3.9 m, m, most of the vertical force values for for single piles N°1 are are to deterd), atpiles, the depth of most of the vertical force values single piles N°1 mine the limited bearing capacity of the pile The ultimate bearing capacity of a pile is greater than thatthat for for pilepile N°4N°4 C3 C3 (F3.9(Fm3.9pilem N°1 > F>3.9Fm3.9pilem N°4 with the the same value ofthe greater than ), with same value of maximum pile N°1 pile C3 N°4),C3 load which load it can carry without orwhen excessive settlement ofdestroyed the ground The test results show imposed on thethe pilefailure head when thethe pilepile has notnot been destroyed (effect load 8mm with the the same loadload imposed settlement has increased suddenly Two load-settlement curves for single mto N°1 Pile Pile m N°4 N°4 the piles in group FN°4 ) with same imposedpiles (cycle m35.59 N°1 m C3, N°4 C3, C1, N°4 C1, C2 N°4) C2 onof the4the pile head This shows sand layers No.1 and No.3 have increased the friction 1) and group piles (cycle 2) shows arethe relatively similar, both increase slowly; P-S curve on pile head This the sand layers No.1 and No.3 have increased the frictionhas a small resistance in the pile group, reducing the load at the depth of m larger in single piles, resistance in the pile group,load reducing the load at the depth of m larger in single piles, angular coefficient before destructive resulting in greater extreme loadload capacity of the pilepile group resulting in greater extreme capacity of the group 139 Table shows thethe comparison of axial forces at the depths of 3.9, 6.5 6.5 andand 8.0 8.0 m m Table shows comparison of axial forces at the depths of 3.9, according to different loadload levels, in order to compare the the loadload distributed intointo the the soilsoil according to different levels, in order to compare distributed through friction between single piles andand piles in 4-pile group DueDue to different loadload through friction between single piles piles in 4-pile group to different levels between single piles andand pilepile group, in this comparison, the the comparison is made levels between single piles group, in this comparison, comparison is made Trung, L T., et al / Journal of Science and Technology in Civil Engineering Limited load capacity for single piles (PUltimate N◦ 1) is 10 T, for 4-pile group (PUltimate N◦ 4) is 49.5 T, calculated pile group coefficient is 1.2375 The settlement of single piles is smaller than of the pile group as destructive, these two values of settlement are recorded as relatively large (31.15 mm and 35.59 mm), which is also consistent with the characteristics of friction piles (it is not good for pile tip to rest on the soil layer) Normally for friction piles, in cohesive soil, the pile group coefficient is usually smaller than 1, however in this case of testing, even if it is a friction pile, due to the influence of sand layer, the pile group coefficient is 1.2375 (greater than 1) It can be assessed that sandy soil layer and porous sand layer (layer 3) have affected the load bearing capacity of the pile group This will be more evident in the results of load distribution by depth Fig shows the axial load transmission curve (F-z) in single piles and pile groups during the loading process Due to some objective and subjective reasons when installing the gauge and lowering the piles, some gauges have errors during the test and fail to measure the deformation of piles (at the position of m for single piles N◦ 1, position of 3.9 m for piles N◦ C1, N◦ C2 and position of 6.5 m for piles N◦ C3), in these cases, the load transmission curve is assumed to be linear through missing positions The F-z curve shows a significant transmission from piles to soil in the range of to m, for both single piles and pile group, the load value at a depth of m is relatively small and close to each other Thus, the load transmitted down to the pile tip is small, which indicates that most of the load has been distributed through friction into the soil These results show the tip resistance has been mobilized to a minimum (near zero), and the load imposed on the pile head is distributed (subjected to) by the axial friction resistance component In the depth from to 3.9 m, the F-z curve has the largest slope, the slope decreases at the area from 3.9 m to m, indicating that friction resistance is mobilized much at the area from to 3.9 m Comparing results of vertical force of single pile N◦ and pile N◦ C3 (figures a and d), at the depth of 3.9 m, most of the vertical force values for single piles N◦ are greater than that for pile N◦ C3 (F3.9 m pile N◦ > F3.9 m pile N◦ C3 ), with the same value of load imposed on the pile head when the pile has not been destroyed (effect load < 100 kN), this shows that friction resistance in the area from to 3.9 m for the pile group has been raised more, reducing the load at the depth of 3.9 m in the pile group This shows the medium-tight sand layer has increased the friction resistance in the pile group compared to the single piles Comparing single piles N◦ and N◦ C1, N◦ C2, N◦ C3, at the depth of m, the non-destructive vertical force values of single piles are greater for the piles in group (F8 m N◦ Pile > F8 m N◦ C3, N◦ C1, N◦ C2 ) with the same load imposed on the pile head This shows the sand layers No and No have increased the friction resistance in the pile group, reducing the load at the depth of m larger in single piles, resulting in greater extreme load capacity of the pile group Table shows the comparison of axial forces at the depths of 3.9, 6.5 and 8.0 m according to different load levels, in order to compare the load distributed into the soil through friction between single piles and piles in 4-pile group Due to different load levels between single piles and pile group, in this comparison, the comparison is made between the load level of 20 kN of single piles with the load level of 22.5 kN of piles in the pile group, load levels 40, 60, 80 kN of single piles at the same load level with 45, 67.5, 90 kN of the piles in the corresponding pile group The results show that with the same load level, vertical force at the same depth, vertical force in the pile group is smaller than that of single piles, this shows that friction resistance in the pile group has been mobilized more in single piles, clarifying the effect of sand layers No and No on the load bearing capacity of piles in the pile group 140 Trung, L T., et al / Journal of Science and Technology in Civil Engineering Table Force along piles at different depths, with different load levels, comparison between single piles N◦ and piles N◦ C1, N◦ C2, N◦ C3 Depth 3.9 6.5 8.0 Notation F3.9 kN F6.5 kN F8.0 kN Depth 3.9 6.5 8.0 Notation F3.9 kN F6.5 kN F8.0 kN Single C1 C2 C3 Single C1 C2 C3 20 6.22 5.65 3.39 22.5 6.22 3.96 22.5 2.26 2.83 22.5 5.65 4.52 40 24.31 18.09 13.57 45 11.31 9.05 45 13 9.05 45 12.44 9.61 Single C1 C2 C3 Single C1 C2 C3 60 27.70 22.05 17.53 67.5 15.83 13.57 67.5 25.44 19.22 67.5 19.79 15.83 80 38.44 31.09 26.01 90 23.74 19.79 90 31.66 25.44 90 30.53 22.61 Conclusions The result of in-situ tests on single piles and group of piles, with multi-layered foundation, including sandy and clayey soil, for friction piles, based on analysis of the test results, it is possible to give a preliminary conclusion as follows: - For multi-layered foundation, when there are sand layers in the foundation, the pile group coefficient may be greater than 1, specifically in case of the study, the pile group coefficient is 1.2375 - For multi-layered foundation, the settlement of pile group is also greater than that of single piles, and the settlement of friction pile is relatively large, specifically in this study, the settlement of single piles as destructive is 31.15 mm and of pile group is 35.59 mm - Sand layers in the foundation greatly affect the load capacity of the pile group, increase ability to mobilize friction resistance in the pile group, increase the load capacity of piles and medium-tight sand layer has bigger level of mobilization friction increase References [1] Poulos, H G (1968) Analysis of the settlement of pile groups Geotechnique, 18:449–471 [2] Randolph, M F., Wroth, C (1979) An analysis of the vertical deformation of pile groups Geotechnique, 29(4):423–439 [3] Randolph, M F., Wroth, C P (1978) Analysis of deformation of vertically loaded piles Journal of Geotechnical and Geoenvironmental Engineering, 104:1465–1488 [4] Poulos, H G., Randolph, M F (1983) Pile group analysis: a study of two methods Journal of Geotechnical Engineering, 109(3):355–372 [5] Poulos, H G (1989) Pile behave – theory and application Geotechnique, 39:365–415 [6] Randolph, M F (2003) Science and empiricism in pile foundation design Geotechnique, 53(10):847– 875 [7] Thuy, D D., Hung, P Q (2015) Verification of the neutral plane method for calculation settlement of pile group Journal of Science and Technology in Civil Engineering (STCE)-NUCE, 9(1):62–68 (in Vietnamese) [8] Ismael, N F (2001) Axial load tests on bored piles and pile groups in cemented sands Journal of Geotechnical and Geoenvironmental Engineering, 127(9):766–773 141 Trung, L T., et al / Journal of Science and Technology in Civil Engineering [9] Al-Mhaidib, A I (2006) Experimental investigation of the behavior of pile groups in sand under different loading rates Geotechnical & Geological Engineering, 24(4) [10] Deb, P., Pal, S K (2016) An experimental and numerical study on behaviour of single pile and group of piles in layered soils under vertical load International Journal of Engineering Research & Technology (IJERT), 5(3):200–208 [11] Elsamny, M K., Ibrahim, M A., Gad, S A., Abd-Mageed, M (2017) Experimental evaluation of bearing capacity and behaviour of single pile and pile group in cohesionless soil International Journal of Engineering Research & Technology (IJERT), 6(5):695–754 [12] Yudiawati, Y., Mochtar, I B., Mochtar, N E (2019) Group capacity and efficiency of full friction piles on very soft soil International Journal of GEOMATE, 16(57):201–208 [13] Tomlinson, M., Woodward, J (2008) Pile design and construction practice Fifth edition, Taylor & Francis 142 ... piles in a group In all cases, elastic and consolidation settlement of a pile group is larger than the settlement of a pile that bears the same load as on individual pile in a group This is because... pile group effect found ranged from 50% to 70% According to Tomlinson and Woodward [13], axial load capacity of a pile group may be significantly smaller than total load capacity of individual... curve according to capacity the capacity depth (F-z) These sandy soil layer and porous sand layer (layer 3) have affected the load bearing sandy and porous (layer 3) have affected the load bearing